I think Discord is great, as long as you account for its shortcomings:
- It should be treated as ephemeral - up to the point of "the service could completely shut down tomorrow"
- It should not be treated as fully private or secure
I love how snappy it is (I never have to wait minutes for messages to load, as I do in Matrix), and also how there are a ton of Lemmy users who have Discord open in the background anyway. While I wouldn't ever propose moving something like the main Lemmy communication channels to Discord, I do think it's much easier to have IMs with regular Lemmy users on Discord than it is to do so on Matrix (as in my experience, most regular users do not have a Matrix client installed).
By the way, lemm.ee also has a Discord server. It's only treated as a secondary channel, all announcements are posted in our meta community on Lemmy itself, and just linked to from Discord, but it's still nice to have IMO.